X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FOrdList.lhs;h=328a5c142220d9e651dcec56987ccfaa05e77938;hb=fa6c4bf01427a4191a595afecf90d96b27bad306;hp=7f22b38e495a0af306f935f7b9aa8fa8a6696cfe;hpb=0065d5ab628975892cea1ec7303f968c3338cbe1;p=ghc-hetmet.git

diff --git a/compiler/utils/OrdList.lhs b/compiler/utils/OrdList.lhs
index 7f22b38..328a5c1 100644
--- a/compiler/utils/OrdList.lhs
+++ b/compiler/utils/OrdList.lhs
@@ -1,4 +1,5 @@
 %
+% (c) The University of Glasgow 2006
 % (c) The AQUA Project, Glasgow University, 1993-1998
 %
 
@@ -19,8 +20,9 @@ infixl 5  `snocOL`
 infixr 5  `consOL`
 
 data OrdList a
-  = Many [a]
-  | Two (OrdList a) (OrdList a)
+  = Many [a]        -- Invariant: non-empty
+  | Two (OrdList a) -- Invariant: non-empty
+        (OrdList a) -- Invariant: non-empty
   | One  a
   | None
 
@@ -35,14 +37,14 @@ concatOL :: [OrdList a] -> OrdList a
 
 nilOL        = None
 unitOL as    = One as
-snocOL as b  = Two as (One b)
-consOL a  bs = Two (One a) bs
-concatOL aas = foldr Two None aas
+snocOL None b    = One b
+snocOL as   b    = Two as (One b)
+consOL a    None = One a
+consOL a    bs   = Two (One a) bs
+concatOL aas = foldr appOL None aas
 
-isNilOL None        = True
-isNilOL (One _)     = False
-isNilOL (Two as bs) = isNilOL as && isNilOL bs
-isNilOL (Many xs)   = null xs
+isNilOL None = True
+isNilOL _    = False
 
 appOL None bs   = bs
 appOL as   None = as
@@ -76,8 +78,9 @@ fromOL ol
         flat None      rest = rest
         flat (One x)   rest = x:rest
         flat (Two a b) rest = flat a (flat b rest)
-	flat (Many xs) rest = xs ++ rest
+        flat (Many xs) rest = xs ++ rest
 
 toOL :: [a] -> OrdList a
+toOL [] = None
 toOL xs = Many xs
 \end{code}